Wireless cellular communication networks are known in which the area covered by the networks are divided into cells. Each cell is provided with a base station which is arranged to communicate with mobile stations or other user equipment in the cells associated with the base station.
In these known systems, a channel is allocated to one user. For example, in the case of the GSM (global system for mobile communications) standard, a user is allocated a given frequency band and a particular time slot in that frequency band. A single information stream from a single user can use that allocated frequency band and time slot. So-called third generation standards are currently being proposed which use code division multiple access (CDMA). In this proposed standard, a user is allocated a particular spreading code to define a channel.
In the currently proposed systems, a single user is allocated a channel as defined by the frequency band, time slot and/or spreading code in a particular cell. Different users may be allocated the same frequency, time slot and/or spreading code but in different cells.
It has been proposed to improve the capacity of a cellular system by using a spatially multiplexed system. In this proposed system, the data rate can be increased by transmitting independent information streams from different (but adjacent) antennas but using the same channel as defined by frequency, time slot and/or spreading code.
In order to successfully receive the different information streams which are transmitted in parallel, it is necessary that the receiving end also have a number of antennas. In practice, for such a system to work, the number of propagation paths for each signal needs to be relatively high, that is high enough to prevent the channel matrix H, the mathematical representation of the transmitted signals through space, from being singular. Typical conditions that lead to Rayleigh Fading, a time related model of the environment which incorporates fluctuations, and spacing of antennae of more than half a wavelength (for GSM with a carrier frequency of approximately 900 MHz, the wavelength is approximately 3.3 mm, and therefore the spacing of the antennae is required to be greater than 1.7 mm) are generally sufficient for non-singularity of the channel matrix.
At the receive end, the detection of the transmitted signals is complicated by the presence of a significant amount of cross talk. In particular, each receiving antenna will receive a linear combination of the transmitted signals.
Accordingly, in order to determine the signals which have been transmitted, each of these different signals need to be extracted from the received combined signal, decoded and interleaved.
It has been proposed to decode the received signals by using the maximum likelihood (ML) approach. This is a brute force method and works by testing every possible combination of substream symbols one by one to determine the one possible combination with the greatest likelihood. This algorithm is possible with some simple cases particularly where the number of parallel information streams is relatively small. However, if the number of streams is relatively large, there are problems associated with the use of this approach. In particular, the algorithm increases in complexity at an exponential rate as the number of parallel data streams increases.
Another method is the ordered successive interference cancellation (OSIC) algorithm. The OSIC algorithm attempts to extract each independent stream individually, by examining the received signals and ordering the independent data streams from highest to lowest signal to noise ratio. These estimates are then used to calculate the interference created by the current estimated data stream which is then used to filter the received signals. This effectively cancels the effect of the estimated data stream from the received signals. This is continued until all of the received signals contain nothing but background noise and all of the independent data streams are decoded.
The OSIC method and its related recoding methods are much simpler than the maximum likelihood approach. However, the OSIC method has the disadvantage that the error performance figures are not as good as the maximum likelihood method.